3 Description

The function Iβiai,bi, also known as the incomplete beta function is calculated using S14CCF.

The input arrays to this routine are designed to allow maximum flexibility in the supply of vector parameters by re-using elements of any arrays that are shorter than the total number of evaluations required. See Section 2.6 in the G01 Chapter Introduction for further information.

On entry: IFAIL must be set to 0, -1​ or ​1. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction for details.

For environments where it might be inappropriate to halt program execution when an error is detected, the value -1​ or ​1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, because for this routine the values of the output parameters may be useful even if IFAIL≠0 on exit, the recommended value is -1. When the value -1​ or ​1 is used it is essential to test the value of IFAIL on exit.

On exit: IFAIL=0 unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry IFAIL=0 or -1, explanatory error messages are output on the current error message unit (as defined by X04AAF).

Note: G01SEF may return useful information for one or more of the following detected errors or warnings.

Errors or warnings detected by the routine:

IFAIL=1

On entry, at least one value of BETA, A, B or TAIL was invalid.
Check IVALID for more information.

IFAIL=2

On entry, array size=value.
Constraint: LTAIL>0.

IFAIL=3

On entry, array size=value.
Constraint: LBETA>0.

IFAIL=4

On entry, array size=value.
Constraint: LA>0.

IFAIL=5

On entry, array size=value.
Constraint: LB>0.

IFAIL=-999

Dynamic memory allocation failed.

7 Accuracy

The accuracy is limited by the error in the incomplete beta function. See Section 7 in S14CCF for further details.

8 Further Comments

None.

9 Example

This example reads values from a number of beta distributions and computes the associated lower tail probabilities.